The invention described in the following paragraphs is directed to the field of spectroscopy, and in particular to a spectral disperser that disperses a spectrum of light received from one or more targets within a field of view.
The science of spectroscopy and designers of spectrometers provide powerful tools to help the military, environmentalists, foresters, urban planners, farmers, miners, etc., classify features, navigate, track objects, measure productivity and yield, and identify trends and objects in the field. Spectrometers can be used to image a scene across a large number of discrete spectral bands such that a complete reflectance spectrum or signature is obtained. These images often are collected and represented as an image cube with multiple slices, with each slice representing a view of the image at a different wavelength. All objects, geological features, water, vegetation, structures, vehicles, metals, paints, fabrics, etc., create a unique spectral fingerprint that can be used to identify the object using known techniques.
The basic technique includes spreading light out into its constituent wavelengths, focusing the different wavelengths on a sensor positioned at an image plane and analyzing characteristics of the images, including the intensity of each wavelength and the wavelength distribution. These techniques can be used, for example, to automatically identify military targets, to separate an incoming missile""s signature from a burning ground signature, to survey crop health, to find camouflaged tanks hidden in the crops, and to identify thermal emissions and hazardous waste, just to name a few uses of this technology.
The multi-spectral information may be gathered using two-color detectors, beam splitters and filter wheels, each of which has its own advantages and disadvantages. For example, two-color detectors have the advantage of instantaneous detection of two different wavebands. Of course, multi-spectral detection with a two color array is limited to two wavebands. In addition, two-color detectors are difficult to design and manufacture, thereby limiting the availability of two-color detectors. Consequently two-color detectors generally are more expensive than panchromatic detectors.
The advantage of using a filter wheel is that only one panchromatic detector is needed. However, a disadvantage of filter wheels is their complexity and cost, and the temporal separation of color channels. Since different color channels must be sampled at separate points in time, collecting data over many wavebands is time consuming and requires a large number of filters on the wheel.
Beam splitters allow instantaneous sampling of information across the bands, but require a detector for each waveband. For cooled infrared detectors, for example, this can dramatically increase the cost, volume, weight, etc., because of the electronics and cooling system that must be hooked up to each detector. This practically limits the number of wavebands that can be sampled. In addition, alignment can be very tricky for the detectors, and packaging limitations can preclude using beam splitters altogether. Beam splitters also are relatively expensive to use for panchromatic sampling.
Some of these devices require changing the distance between an optical element that disperses the light at axially spaced focal planes and a sensor that detects the spectral images. However, it may be difficult to precisely position the axially moveable elements, or to reliably repeat those positions over time, or both. Furthermore, since each wavelength from the continuum of possible wavelengths is in focus at a different axial position along the path of the light, not only is it difficult to capture each wavelength in focus, but the images at different wavelengths have different degrees of magnification. Differences in magnification between images hinders analysis of the images.
These devices require measurements separated in time to obtain a complete spectral signature because at least component must move axially to bring the spectral images at different wavelengths into focus on the sensor. Systems with axially movable elements generally also are less robust and are more sensitive to vibration. Yet another problem with prior devices is that capturing a series of images at different wavelengths over time makes it difficult to obtain a complete image cube if the objects in the field of view are moving.
To avoid some of these problems, diffractive optics have been designed that can sample all of the spectral information at once on a monochromatic imaging array. Light is diffracted into various orders onto the imaging plane and tomography techniques are used to extract the spectral signatures for each imaging point. This type of system has had the disadvantage of lower resolution compared to a monochromatic system of a fixed color separation.
The present invention provides an optical system with a variable dispersion that allows multi-spectral information to be collected from the field of view without having to resort to a two-color detector, beam splitter, or a filter wheel. In a multi-spectral mode, each of the images in different wavelengths are displaced from an image in a central wavelength, and are in focus at a common magnification on a common image plane. Since the amount of wavelength separation is variable and user controlled, the system also can be operated in a non-dispersed xe2x80x9cwhite lightxe2x80x9d mode when multiple wavelengths will be in focus at the same place on the image plane at the same magnification. With proper algorithms, the target signature can be extracted from one or more images for comparison to the signatures of known objects. The variable dispersion provided by the present invention not only allows for selective variation in the amount of dispersion of the incident wavelengths, it also allows for selective variation in the orientation of the dispersion across the image plane. In other words, the xe2x80x9csmearxe2x80x9d of wavelength separation can be made to rotate to a different orientation.
More specifically, the present invention provides an optical system capable of variably dispersing incident electromagnetic energy. The system includes at least two optical elements spaced apart a fixed distance along an optical path. Rotation of one or more of the optical elements relative to one or more of the other optical elements changes the degree of dispersion, and rotation of all of the optical elements together in a common direction changes the orientation of the dispersion.
In accordance with one embodiment of the invention, the system functions as a variable disperser and the optical elements are selected to have approximately zero-degree deviation of a central wavelength and nonzero-degree deviation of at least one other wavelength. The optical elements are grouped into sets, a first set of optical elements and a second set of optical elements. Each set has at least two optical elements that maintain a constant orientation relative to each other. Each set of optical elements includes at least two prisms secured together and aligned so that the central wavelength of electromagnetic energy incident on the set of optical elements generally passes through both prisms.
More particularly, the first set of optical elements includes a first prism formed of a first material and having a first apex angle, and a second prism formed of a second material and having a second apex angle. The second prism has an inverted orientation relative to the first prism. The second set of optical elements includes a third prism formed of a third material and having a third apex angle, and a fourth prism formed of a fourth material and having a fourth apex angle. The third prism has an inverted orientation relative to the fourth prism. Each prism is selected so that a desired central wavelength has approximately zero deviation upon passing through the respective set of optical elements. The first prism is the same as the third prism, and the second prism is the same as the fourth prism.
A motor may be connected to at least two sets of optical elements, each set having at least two optical elements, each set of optical elements being rotatable independently of the other set. The motor includes a first motor connected to a first set of optical elements and a second motor connected to a second set of optical elements.
In accordance with another embodiment of the invention, the system functions as a multispectral scanning system and the optical elements are selected to have nonzero-degree deviation of a central wavelength and zero dispersion. A motor may be connected to each optical element to rotate each optical element independently. In particular, the present invention provides a system that includes four optical elements, each optical element being independently rotatable. Selectively rotating the optical elements can change the look-angle of the system.
In addition, the optical elements may include one or more of a diffractive device (e.g., at least one diffraction grating), a refractive device (e.g., at least one prism) or both. The system may further include an imaging assembly (such as a lens doublet) at a fixed distance from the optical elements that receives dispersed electromagnetic energy from the optical elements and focuses the electromagnetic energy toward an image plane. In addition, the system may include a detector at the image plane that is a fixed distance from the optical elements to receive at least a portion of the electromagnetic energy from the imaging assembly and to convert the incident electromagnetic energy into electronic data. The detector may include at least one photodetector pixel or an array of photodetector pixels.
The system may further include at least one of a processor for analyzing the electronic data from the detector and a memory for storing the electronic data. The processor analyzes the electronic data to identify spectral signatures of objects within the system""s field of view. The memory includes a library of electronic data representative of spectral signatures of known objects to facilitate identification of objects in a field of view. The system also may include a controller that selectively and independently controls each motor. The system provided by the present invention has many uses, including such diverse applications as in a missile or a projector, for example.
The foregoing and other features of the invention are hereinafter fully described and particularly pointed out in the claims, the following description and annexed drawings setting forth in detail a certain illustrative embodiment of the invention, this embodiment being indicative, however, of but one of the various ways in which the principles of the invention may be employed.